Cretaceous lacustrine stromatolites in the Gyeongsang Basin (Korea): Records of cyclic change in paleohydrological condition

초록

Textural observation and stable isotope analyses of middle Cretaceous lacustrine stromatolites of the Sinyangdong Formation, Gyeongsang Supergroup (Korea) reveal that the alternating laminations preserve the cyclic change of paleohydrological conditions influenced by local paleoclimate. These alternating laminations are repetitive, consisting of a lower fibrous calcite and an upper microbial micrite layer forming a couplet, which is individually about 2-5 mm thick. The lower-to-upper boundaries are always gradational, but upper-to-lower boundaries are sharp and abrupt, reflecting not only cessation of growth but also subaerial dissolution. A delta(18)O enrichment from fibrous calcite to microbial micrite is consistent, indicating lowering of paleolake level mainly due to strong evaporation during the dry period when the stromatolite grew. Simultaneous and coeval depletion in delta(13)C from fibrous calcite to microbial micrite suggests that more available are C-12 atoms provided by the decay of organic matter due to the decreasing paleoproductivity and lowering lake level. These systematic trends of texture and isotopic composition may be intrinsically linked to local paleoclimate change.

키워드